1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for positioning a mobile station within a cellular network, and specifically to positioning a mobile station during a handover.
2. Background of the Present Invention
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. A standardization group, European Telecommunications Standards Institute (ETSI), was established in 1982 to formulate the specifications for the Global System for Mobile Communication (GSM) digital mobile cellular radio system.
With reference now to FIG. 1 of the drawings, there is illustrated a GSM Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC 14 provides a circuit switched connection of speech and signaling information between a Mobile Station (MS) 20 and the PLMN 10. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which the MS 20 may move freely without having to send update location information to the MSC 14 that controls the LA 18. Each LA 18 is divided into a number of cells 22. The MS 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless.
The MSC 14 is in communication with at least one Base Station Controller (BSC) 23, which, in turn, is in contact with at least one Base Transceiver Station (BTS) 24. The BTS is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the cell 22 for which it is responsible. It should be understood that the BSC 23 may be connected to several BTS""s 24, and may be implemented as a stand-alone node or integrated with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a whole, are generally referred to as a Base Station System (BSS) 25.
The MS 20 and the BTS 24 communicate over a radio interface, which utilizes the Time Division Multiple Access (TDMA) concept, with one TDMA frame per carrier frequency. The TDMA frames are numbered in a cyclic pattern. Each TDMA frame consists of a number of time slots, in which each time slot is referred to as a physical channel. Depending upon the type of information being transmitted, different types of logical channels are mapped onto these physical channels. For example, to transmit speech, the logical channel xe2x80x9ctraffic channelxe2x80x9d must be mapped onto one of the physical channels. The information sent on one of these channels is called a burst.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information, for subscribers registered within that PLMN 10. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
Determining the geographical position of an MS 20 within a cellular network 10 has recently become important for a wide range of applications. For example, location services (LCS) may be used by transport and taxi companies to determine the location of their vehicles. In addition, for emergency calls, e.g., 911 calls, the exact location of the MS 20 may be extremely important to the outcome of the emergency situation. Furthermore, LCS can be used to determine the location of a stolen car, for the detection of home zone calls, which are charged at a lower rate, for the detection of hot spots for micro cells, or for the subscriber to determine, for example, the nearest gas station, restaurant, or hospital, e.g., xe2x80x9cWhere am Ixe2x80x9d service.
As can be seen in FIG. 2A, which will be described in connection with the signaling diagram in FIG. 2B of the drawings, upon the reception of a positioning request from a Location Services (LCS) client (step 200), the MSC 14 sends a MAP_PERFORM_LOCATION message to a Serving Mobile Location Center (SMLC) 270 within the PLMN 10 associated with the MSC 14 (step 205). The SMLC 270 is responsible for carrying out the positioning request and calculating the MS 20 location. It should be noted that more than one SMLC 270 may be located within each PLMN 10. Thereafter, the SMLC 270 determines the positioning method to use (step 210), and if the Time of Arrival (TOA) positioning method is selected, the SMLC 270 returns a MAP CHANNEL INFORMATION message to the MSC 14 (step 215). The MSC 14, in turn, forwards a BSSMAP CHANNEL INFORMATION message to the serving BSC 23, requesting the physical channel description of the traffic channel that will be used to perform a positioning handover (step 220). The message also includes information on the cell 22 ID""s and TDMA frame numbers for the serving and candidate cells 22 to which positioning handovers are to be performed to, along with a value for a delta timer 27.
In response, the BSC 23 sends a BSSMAP CHANNEL INFORMATION ACK message to the MSC 14, which includes the requested physical channel description (step 225). In addition, the BSC 23 starts the delta timer 27 (step 230). The MSC 14 forwards this physical channel description received from the BSC 23 to the SMLC 270 (step 235), which uses this physical channel description to configure at least three Location Measurement Units (LMUs) 260 (only one of which is shown) within the PLMN 10. The LMUs 260 are responsible for obtaining positioning measurements and providing these measurements to the SMLC 270 for use in calculating the location of the MS 20. All communication to and from the LMUs 260 are sent over the air interface. Therefore, each LMU 260 is in wireless communication with an associated BTS 24. The SMLC 270 selects which LMUs 260 should obtain the positioning measurements (step 240), and sends LCS Information Request messages to each of these selected LMUs 260 (step 245).
At the expiration of the delta timer 27 within the BSC 23 (step 250), the BSC 23 begins the positioning handover process. The delta timer 27 is used to ensure that the SMLC 270 has enough time to configure the LMUs 260 prior to initiating the positioning handover process. A positioning handover occurs when the BSC 23 sends a HANDOVER (HO) COMMAND message to the MS 20 (step 255), instructing the MS 20 to perform a handover to the serving BTS 24 or a target BTS (not shown) on a specified channel. The HO COMMAND message also indicates the TDMA frame number that the MS 20 should begin sending access bursts. When the MS 20 starts sending the access bursts in a HANDOVER ACCESS message (step 260), the configured LMUs 260 measure the Time of Arrival (TA) of these access bursts (step 265). Since the handover is a positioning handover, and not a radio-related handover, the BTS 24 will not respond to the HANDOVER ACCESS message, and the MS 20 will stop sending the access bursts upon the expiration of a timer (not shown) within the MS 20. Thereafter, the MS 20 returns to the old channel that it was assigned to, and sends a HANDOVER FAILURE message to the BSC 23 (step 270).
These TOA measurements are forwarded from the LMUs 260 to the SMLC 270 (step 275) for use in assisting the calculation of the geographical location of the MS 20 (step 280). After the SMLC 270 calculates the MS 20 location (step 280), this location is transmitted to the LCS client 280 that requested the positioning (step 285). It should be noted that the requesting client 280 could be located within the MS 20 itself, within the MSC 14 or could be an external node, such as an Intelligent Network (IN) node. If the client 280 is not within the MS 20 or within the MSC 14, the location information is sent to the requesting client 280 via the MSC 14 and a Gateway Mobile Location Center (GMLC) 290.
However, if a radio-related handover is initiated before the expiration of the delta timer 27, in today""s architecture, the BSC 23 cancels the ongoing positioning, and notifies the MSC 14 of the cancellation of the TOA request. A radio-related handover may occur if the MS 20 is involved in a call connection, and roams into the coverage area of a new cell 22. In order to continue the call, the call must be handed over to the new cell 22. This handover process may take several seconds, which could cause the MS 20 to start transmitting the access bursts for the positioning handover in the wrong TDMA frame number. Since the LMUs 260 are listening at the time associated with the same TDMA frame number that was sent to the BSC 23 in the BSSMAP CHANNEL INFORMATION message, the LMUs 260 will not be listening to the access bursts sent by the MS 20. Therefore, in order to avoid this situation, the BSC 23 cancels the positioning. This could potentially delay the positioning process anywhere from three to five seconds, which may be undesirable for many LCS clients 280.
The present invention is directed to telecommunications systems and methods for allowing an ongoing Time of Arrival (TOA) based positioning of a mobile station to proceed even if a radio-related handover has occurred before the expiration of a delta timer in a Base Station Controller (BSC). The BSC stores an HOTIMER value therein, which corresponds to the amount of time involved in a normal radio-related handover procedure from the sending of a Handover Command to the reception of a Handover Complete message. If the BSC delta timer is active, e.g., positioning is ongoing, and the BSC determines that a radio-related handover is needed, the BSC checks the amount of time remaining in the BSC delta timer. If that amount is more than the HOTIMER, then the ongoing positioning is not canceled.